Frequency comparison apparatus



j\ amh 9, 194

Original Filed April 15, 1945 2 Sheets-Sheet 1 Q m. g 5E6 62.363 5655 0 0 0 65335 0 W INVENTOR LOUIS F. MAYLE Patented Mar. 9, "1948 FREQUENCY COMPARISON APPARATUS Louis F. Mayle, Fort Wayneylnd assignor, by mesne assignments, to Farnsworth Research Corporation, a corporation of lndiana Substituted for abandoned application- Serial No. 483,139, April 15, 1943. This application August 7, 1946, Serial No. 688,852

7 Claims. 1

This invention relates to automatic frequency control apparatus and particularly to such apparatus used in connection with a television timer.

This application is a substitute for abandoned application of Louis F. Mayle, Serial No. 483,139, filed April 15, 1943, for Frequency comparison apparatus.

According to conventional television practice it is customary to employ timer apparatus for definitely correlating the frequency of the syn-- chronizing signals with a source of standardizingfrequency such as a commercial alternating current power supply. The timer apparatus includes an oscillator operating at a frequency of twice that of horizontal scanning (where interlaced scanning is employed), and a timer chain or frequency divider. By means of the latter device the high frequency oscillations are converted into control impulses at a frequency suitable to synchronize the scanning generators of the transmitter and to provide synchronizing signals to combine with the video signal to be transmitted.

In order to control the frequency of. the oscil-- la'tor so that the frequency of the oscillations is related at all times to a standardizing frequency, there are also derived from the frequency divider relatively low frequency impulses which are compared with the frequency of impulses, or of a sinusoidal wave, of substantially the same frequency which are derived from the standardizing source. Any deviation of the compared frequencies is detected and a control voltage is generated representative of the deviation. Theron-- trol voltage is utilized to adjust the frequency of the oscillator to restore i'tto the desired relationship with the standardizing frequency.

As is well known in the art, however, numerous arrangements of electronic apparatus have been employed for making the frequency comparison and for generating the control voltage; Many of these devices are considerably complicated and do not accomplish the desired result in the most effective manner.

An object of the present invention, therefore, is to provide improved means for detecting frequency deviations from a standardizing frequency, whereby to control the frequency of a high frequency oscillator employed in a television timer.

Another object of the invention is to provide: an improved frequency comparison device, where-- by to facilitate the generation of control voltages for the frequency determining circuit of an oscillator employed in a television timer.

In accordance-with the present invention there is provided a multi-grid electronic device connected. for operation as a trigger'device. Two series of impulses are applied-to respective control grids of the electronic device in opposite polarity and normally displaced in phase by Oneseriesof impulses is derived from television timer apparatus and the other series of impulses is derived from a'source of standardizing frequency. The effect of the oppositely poled impulses applied to the control grids of the electronic device is to render it alternately more conducting and less conducting. The auxiliary grid elements of the device are interconnected in'a manner such that the device is changed from a more conducting to alessconducting state or vice versa substantially instantaneously.

The abrupt Voltage changes appearing at the anode of the-electron device are impressed upon a rectifier'circuitfunctioning as a voltage doubler. Solong as the frequencies of the two series of impulses applied to the control circuits of the electronic trigger de'vice'are' the same and are applied 1'80 out of phase with each other, the voitag'e doubler producesno control voltage. However; there is generated by this apparatus, in response to a deviation of the oscillator-derived frequency from the standardizingfrequency a control-voltage representative of the deviation. This control voltage is applied to the input circuit ofan automatic frequency control circuit which ie-connected to" the frequency determining circuit of the high frequency oscillator, whereby to adjust the frequency of this device to reestablish the desired relationship with the standardizing frequency.

For a' better understanding of the invention together with otherand' further objects thereof, refere'nc'e'is had to the following description taken in connection with the accompanying drawings,

and itslscope'w'ill be pointedout in the appended claims.

In the accompanying drawings,

Fig. I is a circuit diagram of frequency comparison apparatus embodying the instant invention;

Fig. 2 isa graphicalrepresentation of a char-' acteristicof the electronic discharge device used in thefrequency comparator; and,-

Figs. 3, 4=and- Erareo'ther graphical representations showing the relationship between the various voltages under different conditions.

Referring now to Fig; I of the drawings, there isshown'an osc'illator 12' for generating energy a-t-a relatively'high frequency. Theoutput circuit of the 1 oscillatori'i's connected to a television timer chairr or'fre'quency divider I32 A connection is output circuit of the impulse generator is connected through a coupling condenser |8 to a second control grid IQ of the tube I6.

Associated with the tube I6 is a source of direct current potential, such as a battery 20. Connected across tse terminals of the battery 20 is a voltage divider comprising a series connection of resistors 2|, 22, 23, 24 and 25. Various electrodes of the tube |6 are connected to difierent points on the voltage divider. The control grid I9 is connected by a resistor 21 to the junction point between resistors 2| and 22, and the control grid I is connected by a resistor 23 to the junction point between resistors 22 and 23. The' cathode 29 of the tube is connected to the junction point between resistors 23 and 24. The anode 3| is connected through a load resistor 32 to the junction point between resistors 24 and 25.

An auxiliary voltage divider is provided for a purpose to be described and comprises resistors 33, 34 and 35 arranged in a series connection between the negative and positive terminals of the battery 20. The screen grid 36 of the tube I6 is connected to the junction point between resistors 34 and 35. and the suppressor grid 31 is connected to the junction point between resistors 33 and 34.

The anode 3| of the tube l6 also is coupled by means of a condenser 38 to a voltage doubler rectifier circuit. This apparatus includes two diodes 39 and 4| which are arranged in' inverse relationship as shown. There is provided a connection including a resistor 42 from the coupling condenser 38 to the cathode of the diode 39 and to the anode of the diode 4|. A leak resistor 43 for the coupling condenser 38 connects this condenser to ground.

Connected between the anode of the diode 39 and the cathode of the diode 4| is a series arrangement of two identical condensers 44 and 45. The mid-point of this condenser arrangement is connected to ground. The condenser arrangement is also shunted by a series connection of two identical resistors 46 and 41. The mid-point of the resistor arrangement is connected through a filter including series choke coils 48 and 49 and shunt condensers 5| and 52 to a terminating impedance such as a resistor 53.

The voltages developed across the resistor 53 are impressed by means of a circuit including a resistor 54 upon the input terminals of an'automatic frequency control circuit 35. The A. F. C. circuit may take any conventional form which is known to adjust the frequency of the oscillator l2 in accordance with the polarity and magnitude of the voltages developed in resistor 53.

Before considering the operation of the automatic frequency control system as a whole, consideration will be given to two of the components thereof. There is a characteristic of an electrondischarge device having at least a cathode, a control grid. a screen grid. a suppressor grid and an anode that, when the electrodes are connected so that the voltage applied to the suppressor grid is varied in proportion to a variation of the screen grid voltage and in the same direction, the screen grid exhibits a negative resistance in one region of its operation. As seen in Fig. 1, the screen grid 36 and the suppressor grid 31 of the tube l6 are connected to the auxiliary voltage divider in a manner to satisfy the voltage variation condition referred to. The resistors 33, 34 and 35 constitute a load resistance for the screen grid of the tube. As the current drawn by the screen grid increases, the potential of the junction point I negative.

between resistors 34 and 35 become more negative. This potential is the impressed screen grid voltage. As a consequence of the increased negative potential of the junction point between resistors 34 and 35' the potential of the junction point between resistors 33 and 34 becomes more Thispotential is impressed upon the suppressor grid 31 of the tube. Variations in the screen grid voltage in the opposite sense cause similar proportional variations in the opposite sense in the suppressor grid voltages.

Referring now to Fig. 2 of the drawings, there are shown three curves, each representing a variation in the current drawn by the screen grid with variations in the voltage impressed upon the screen grid when the suppressor voltage is varied in a proportionate amount in the same direction. The curve 51 represents this characteristic of the tube when the potentials of the control grids thereof have normal negative values determined by the respective biasing circuits including the resistors 21 and 28. The curve 59 represents this characteristic when the control grid potential is less negative or even in the positive grid region. The curve 58 represents this characteristic when the control grid potential is more negative. These curves are but three of a family of curves which may be plotted for difierent control grid voltages.

Considering the curve 51, it is seen that in the region CA, as the screen voltage increases the screen current also increases. But after reaching the point A further increases in the screen voltage produce decreases in the screen current as shown by the curve in the region AB. The line 60 represents the load line corresponding to the screen grid resistance and may intersect the characteristic curve 51 in three points 6|, 62 and 63 at the same time. Points 6| and 63 represent stable conditions and point 62 an unstable condition. Advantage is taken of this characteristic of the tube so that it is made to operate as a trigger device.

Assume, for example, that normal biasing-potentials are applied to the control grids of the tube and that the screen voltage and current are represented by the point 6| on curve 51. If, now, the control grid potential is made more negative the screen voltage vs. screen current is represented by the curve 58 which is intersected by the load line 60 only at the point 64. When the control grid potentials are restored to normal so that the curve 51 again represents the screen voltage and current characteristics, the intersection of the load line 60 with this curve at the point 63 represents the stable screen voltage which exists until the control grids of the tube are subjected again to potentials other than the normal biasing potentials.

When the control grid voltage is made less negative, or even positive, the screen voltage vs. screen current characteristic is represented by the curve 59 which is intersected by the load line 60 only at the point 65. The restoration of the control grid potentials to normal results in a screen grid voltage and current represented by the point 6| on curve 51. These conditions obtain until there are further changes made in the control grid voltages.

Consequently, if the control grid voltages are alternately made positive and negative, the screen voltage of the tube IS in Fig. 1 may be made to change abruptly from a value indicated by the point 6| of Fig. 2 to a more positive value represented by the point 63 of this figure, and vice versa. Since, by reason of the connection between thescreen grid-:86 of-irE'ig-l andthesuppressor grid 31, the voltages applied tothese grids vary in proportion and inthesamerdirection, the voltage of the suppressor gridlmay be made .alternately negative and positive with respect-to the cathode 29. It may-bemade sufficiently negative to minimize the anode-to-cathode current in thetube and also its positive value may: be .sufficient to effect a maximumzconduction of anodeto-cathcde current. as a-trigger device,-and, since lt isprovided with two control grids, there may be applied to one of the grids positive voltageimpulses derived from one of the frequency sources such as the alternating 1 current power supply as illustrated, and there may be-applied to the other controlgrid negative voltageimpulses derived from the local oscillator.

By the trigger operation of thetubei Hi there are developed-at the anode 3| thereofvoltages which vary abruptly in their positive magnitudes. When the tube is less conducting the potential of the'ancde is at a'maximum positive value and when the tubeis more conductlngthis potential is at a minimum positive value. Thewave form of the anode voltage issubstantially square topped and, when these voltages are impressed upon the voltage doubler circuits, they-produce substantially square topped alternating current voltages in the output circuit of the voltagedoubler. If the voltage developed in theoutput circuit ofthe trigger tube |6 is equivalent-t aseries ,of equal half cycles of opposite-polarity, the voltage which appears between the junction -point.of the resistcrs lfa' and l? andthe junction point of the condensers-it andt will be zero. This may .be seen by considering that, in-response totheequal half cycles applied to diodes 39and4l, eachhas impressed thereon the same'pealrvoltage. Thus, withbcth positive and negative half cycles of curves of Fig. 3. The negative voltage impulses such as 56 are derived from thefreq'uency divider l3 of Fig. land the positive voltage impulses such as El -are derived from the impulseg-en- 'erator l'l of Flg. 1. {Both oftheseseries of-impulses are *illustratedasbeing of the same frequency and are displaced 180 with respect to one another. The curve =68 of Fig. -3-il1ustrates the voltage appearing at the terminals of-the voltage doubler input circuit ofFig. -1. -Itis seen that,coincident Withthe occurrence of the positive impulses 61, a positive half cycle of the curvefitis initiated. This resultsfrom' the triggering 01? of the tube It, whereby the potential of the anode'tl thereof is raised to its maximum positive value. Also, coincident with the occurrence of the negative impulses 66, the positive half cycle of the curve B8 is terminated and the succeedingnegative half cycle is initiated. Under the control of succeeding positive and negative impulsesapplied to the trigger device', the

Thus, ithe tube is :operated succeeding positive and negative .half .icycles of the wave 68 are generated. .ltilsseenthatgby reason of the. phase relationships}:etweenrthe;impulses 55 and 51, the positive and negative .half cycles of the wave '.-68.ar.e of .equaltimeduration. Consequently, in. orderstozbalance. thezpositive. and negative energy, Lthe .:alternating .1 current reference axis .269 .of this wave .115 symmetrically ;1ocated with respect to the .positive and negative voltage peaks.

Fig. 4 ;illustrates .the :operation of the. apparatus when,;by-reason;of.a decrea einthe fr quency of the oscillator, there ,is ,produced a phase shift of :theimpulses of opposite polarity which are applied tothe control gridiof the tube. In order to facilitate the description of the operation of theapparatus it;is ,asswnedthatlithe frequencies of 1 the .two series of impulses are substantially the :samefor adewcycles butthat the impulses areso displaced in phase because of an. earlier. assumed decrease of .oscillatorifrequency that the positive, standardizing gimpulses TH precede .thernegative oscillator-derived ,impulses l2 by-less than The curve :'l3,ill,us-

trates. the voltage appearing at the terminals, of the voltage doublerinputcircuit orFig. :1. By reason of the illustrated and describ.ed phase relation of the positiveanda negative impulsesthe time durationsof the positive.half;-.cycles of;the

voltage wave-13am decreaseduand those: of the negative .half cycles are .increased s;correspondingly. .It will. be .noted,-. however, :that; the,peakto-peak voltage difference -..between the positive and negative :half cycles :is the same as in the previously described. case. of perfect synchronism.

In. order to balance the .positive, and. negative energy. represented bythesourve :13, the. alternating; current reference-axis of thesvoltage. wave. is represented at.'i4. :Thus, it'isuseenzthatthe positive half cycles:have.;greater peak-.values with reference to the alternating. current aXi-s than do the ,negativehalf cycles. :The condensers 44 and -45 are charged to, these, respective peak voltages,

butthe numerical sum. of .the; voltages to which the condensers:are:. charged, remains equal to the peak-toepeak voltage of the wave 13. However-,it is seen thatthe voltage at, the, junction point of resistors 46 and -41 will be positive. with respect to the grounded. Junction, point. of condensers 44 anda45.

Fig. .5 illustrates the functioning of theyoltage doubler apparatus when the oscillator. frequency is increased to produce .a phase. shiftof the negative oscillator derived impulse-s15, with respect to thepositive impulses -16 derived from the alternating current powensupplyeln. this case the negative impulses 15 follow thejpositive impulses 16 by more than 180 'I l1 -iresulting alternating current voltagelwave:is rep-resented by the curve 11 in which itagainis seen that the peak-to-peak voltage of' the p o siti-ve;,and negative half cycles is the. same as in previously described cases. In this instancethetiine duration of the positive half cycles. isgreaterthan that of the negative half :Qycles. Consequently, in order to balancegthe positive and negative energy the alternating current-reference. axis. of the voltage wave 11 is represented at.18. .Now, it is seen that with reference:tothealternating current reference axis the negative Lha'lf cycles have greater peak..-values.than. do thepositive half cycles. Thus, there is developed at the junction point oftheresistorsflll and M of Fig. 1 a negative voltage with respect to the grounded junction point of the condensers-44 and 45.

Referring now to the operation of the frequency comparator apparatus as a Whole, assume that in Fig. 1 the negative impulses derived from the frequency divider l3 are of exactly the same frequency as the positive impulses derived from the impulse generator l1. and that these positive and negative impulses are displaced in phase by 180. These impulses then control the triggering of the tube l6, whereby there is generated in the output circuit thereof a square wave alternating current in which the positive and negative half cycles are of equal time duration. Thus, there is developed by the voltage doubler apparatusa rectified voltage whereby the potential of the junction point of the resistors 46 and 41 is at all times the same as the potential of the junction point of the condensers 44 and 45. In this case there is no voltage developed across the terminating resistor 53, and as a result there is not applied to the input terminals of the automatic frequency control circuit 55 any frequency correcting voltage.

If now, by reason of a frequency deviation between the two sources of impulses, there occurs a phase shift between the positive and negative impulses, the time durations of the positive and negative half cycles of the voltage developed in the output circuit of the tube iii are no longer equal. Consequently, there is developed between the junction point of the resistors 46 and 41 and the junction point of the condensers 44 and 45 a control voltage which has a polarity representative of the sense of the frequency deviation and a magnitude representative of the amount of the frequency deviation. This voltage is, of course, slightly pulsating and, when properly filtered by the apparatus provided for that purpose, appears as a substantially constant direct voltage across the termintaing resistor 53. This constant direct voltage is impressed upon the automatic frequency control circuit 55 to control this apparatus in a manner to restore the oscillator 12 to a frequency having the predetermined relationship to the frequency of the alternating current power supply.

While it will be understood that the circuit specifications of the automatic frequency control apparatus may vary according to the design for any particular application, the following circuit specifications and apparatus employed for the purpose described are included, by way of example only:

Condenser 14-500 micro-microfarads Tube l6-7Q7 Condenser Iii-500 micro-microfarads Battery 20135 volts Resistor 2 I3300 ohms Resistor 22100 ohms Resistor 23-100 ohms Resistor 24-2500 ohms Resistor 25'7500 ohms Resistor 32--l0,000 ohms Resistor 33-l00,000 ohms Resistor 34-50,000 ohms Resistor 35-l00,000 ohms Condenser 38-20 microfarads Resistor 42-1,000 ohms.

Resistor 43--20,000 ohms Condensers 44 and 450.1 microfarad Resistors 46 and ll-100,000 ohms Choke coils 48 and 491,000 henries Condensers and 52-1 microfarad Resistor 53-l00,000 ohms Resistor 54- -2.2 megohms While there has been described what at present is considered the preferred embodiment of the invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and therefore, it is aimed in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What is claimed is:

1. Automatic frequency control apparatus for a television timer comprising, an electronic trigger device for developing an alternating current wave, means for varying the conductivity of said trigger device in response to series of positive and negative impulses, the positive and negative energy content of said alternating current wave being determined by the phase relationship of said positive impulses with respect to said negative impulses, means for developing a control voltage representative of the predominant energy content of said alternating current wave, and means under the control of said control voltage for adjusting the phase relationship between said negative and positive impulses to synchronize said impulses.

2. Automatic frequency control apparatus for a television timer comprising, an electronic trigger device for developing a square wave alternating current, means for varying the conductivity of said trigger device in response to series of positive impulses derived from a source of standardizing frequency and negative impulses derived from said television timer, the positive and negative energy content of said alternating current Wave being determined by the phase relationship of said positive and negative impulses, means including a voltage doubler circuit for developing a control voltage representative of the predominant energy content of said alternating current wave, and means under the control of said control voltage for adjusting the phase relationship between said negative and positive impulses to synchronize said impulses.

3. Automatic frequency control apparatus for a television timer comprising, an electronic trigger device for developing an alternatin current wave, means for rendering said trigger device less conducting in response to a series of positive impulses derived from a source of standardizing frequency, means for rendering said trigger device more conducting in response to a series of negative impulses derived from said television timer, the positive and negative energy content of said alternating current wave being determined by the phase relationship of said positive and negative impulses, means including a voltage doubler circuit for developin a control voltage having a polarity and a magnitude dependent upon the polarity and magnitude respectively of the predominant energy content of said alternating current wave, and means variable under the control of said control voltage for adjusting the phase relationship between said negative and positive impulses to synchronize said impulses.

4. Automatic frequency control apparatus for a television timer comprising, a source of negative impulses, a source of positive impulses. an electronic trigger device having a first control grid responsive to said positive impulses to render said device less conducting and a second control grid responsive to said negative impulses to render said device more conducting, whereby to develop an alternating current wave, the time durations of the positive and negative half cycles of said alternating current wave being determined by the phase relationship between said positive and negative impulses, means controlled by said alternating current wave for developing a control voltage representative of the average time duration of half cycles of one polarity with respect to the average time duration of half cycles of the opposite polarity, and means responsive to said control voltage to vary the frequency of said negative impulses, whereby to maintain the frequency of said negative impulses equal to the frequency of said positive impulses.

5. Automatic frequency control apparatus for a television timer comprising, a source of negative impulses, a source of positive impulses, an electronic trigger device having a first control grid responsive to said positive impulses to render said device less conducting and a second control grid responsive to said negative impulses to render said device more conducting, whereby to develop an alternating current wave, the time durations of the positive and negative half cycles of said alternating current wave being determined by the phase relationship between said positive and negative impulses, a voltage doubler circuit responsive to said alternating current wave for developing a control voltage representative of the average time duration of half cycles of one po1arity with respect to the average time duration of half cycles of the opposite polarity, and a fre-,

quency controlling circuit for said negative impulse sour-ce responsive to said control voltage, whereby to maintain the frequency of said negative impulses equal to the frequency of said positive impulses.

6. Automatic frequency control apparatus for a television timer comprising, a source including an oscillator of negative impulses, a source including an alternating current power supply of positive impulses, an electronic trigger device having a first control grid responsive to said positive impulses to render said device less conducting and a second control grid responsive to said negative impulses to render said device more conducting, whereby to develop an alternating current wave, the amplitudes of the positive and for said oscillator responsive to said control voltage, whereby to adjust the frequency of said oscillator to a definite relationship to the frequency of said alternating current power supply.

'7. Automatic frequency control apparatus for a television timer comprising, a source including an oscillator of negative impulses, a source including an alternatin current power supply of positive impulses, an electronic trigger device having a first control grid responsive to said positive impulses to render said device less conducting and a second control grid responsive to said negative impulses to render said device more conducting, whereby to develop an alternating current wave, the amplitudes of the positive and negative half cycles of said alternating current Wave being equal at all times and the time durations of said positive and negative half cycles being determined by the phase relationship between said positive and negative impulses, a voltage doubler circuit for developing from said a1- ternating current wave a control voltage having a polarity determined by the average time duration of half cycles of one polarity with respect to the average time duration of half cycles of the opposite polarity and a magnitude determined by the magnitude of the difference of said time durations, and a frequency controlling circuit for said oscillator variable in response to said control voltage, whereby to adjust the frequency of said oscillator to a definite relationship to the frequency of said alternating current power supply.

LOUIS F. MAYLE. 

